The density of an object is defined as its mass per unit volume (d=m/v). Density is generally stated in terms of grams per cubic centimeter or pounds per cubic foot. The mass of an object is easily established with a balance. The volume of an object is also easily determined if the object is an impervious solid of simple geometric shape. For example, the volume of a cube is the edge length cubed (L.sup.3).
Determining the volume of an object of complex shape or an object with holes or pores can be difficult and involve time-consuming measurement techniques. Further, the volume of the object is a matter of definition. The volume of an object can be determined by either excluding the volume of the holes and the pores to find its absolute density (also termed the true or skeletal density) or including the holes and pores up to the point at which they break the plane of the surface to determine the envelope density (also called the bulk or apparent density). Absolute density can be determined by compressing the object until all of the voids are eliminated and only a continuous solid phase remains or by a pycnometer employing helium gas that penetrates the pores of the object.
The present invention is directed towards determining the envelope density of rigid, porous objects. Common rigid, porous objects include everything from sugar cubes and aspirin tablets to floor tiles, concrete, and bakery cookies. Other examples include oil well cores (after the liquid therein is expelled), catalyst pellets, and sintered metal bearings and gears.
The envelope density of an object is valuable when used in conjunction with its absolute density to determine the porosity of that object and its specific pore volume (i.e., the pore space that was eliminated upon compression): EQU Porosity=[(1- Envelope Density/Absolute Density) 100]% EQU Pore Volume=[1/Envelope Density - 1/Absolute Density]cm.sup.3 /g
Porosity and pore volume are parameters that frequently establish the fitness of an object for its intended purpose.
Until recent years, the most widely used technique for assessing external volume involved submerging the test object in mercury and measuring the displaced liquid volume. Testing of this type is described in ASTM Standard Test Method C493-93, entitled "Bulk Density and Porosity of Granular Refractory Materials by Mercury Displacement." Mercury is a non-wetting fluid that bridges the pore entrances and does not penetrate small cracks, holes, or pores. The use of mercury, however, is being phased out because of health concerns. The sample object also becomes contaminated by contact with mercury and must be treated as a hazardous waste.
Another known method requires the porous object to be boiled in water and then remain submerged while the water cools and fills the pores. The sample is first weighed dry, then weighed while suspended in water, and weighed after superficial drying to obtain the desired information. This testing method is described in ASTM Standard Test Method C20-92, entitled "Apparent Porosity, Water Absorption, Apparent Specific Gravity, and Bulk Density of Burned Refractory Brick and Shapes by Boiling Water." A related procedure, ASTM Standard Test Method C830-93, substitutes evacuation for boiling in water and then fills the pores with water or mineral spirits. Both of these methods are tedious and require considerable operator skill to dry the exterior surface of the object while keeping the pores filled with the liquid.
A further method seals off the pores of an object by dipping the object in melted paraffin wax. The wax is not supposed to fill the pores but to seal off the pore entrances. The dry weight, the wax-coated weight in air, and the wax-coated weight suspended in water are used to determine the envelope density. See ASTM Standard Test Method C914-89, entitled "Bulk Density and Volume of Solid Refractories by Wax Immersion." This method is also tedious and can destroy the usefulness of the object because the coating may be impractical to remove.
Attempts have been made in the past to measure the envelope density of an object with the use of dry materials. An example includes British Patent No. 108,512 in which the envelope density of a loaf of bread is determined by filling a container with turnip seeds both with and without the bread present. The envelope density of the loaf is defined as the difference in the volume of the turnip seeds present in the container in both tests. Another method is described in German Patent No. 1,959,681 in which the interior volume of a complex cast iron mold cavity is determined. The cavity is filled with a free flowing powder of known density and then the powder is weighed to determine the cavity volume.
Similar methods include the determination of envelope density of bits of silica gel, carbon, and other particles by placing the particles in a container and filling the container and an extension of it with a fine powder of bronze, steel or zinc. The container and extension are then vibrated vertically and the contents compacted. The extension is removed and the volume of the sample particles is determined by measuring the mass of the powder filling the container with and without the particles present. Results are dependent upon the vigor of the vibration, the excess mass of the powder in the cup extension, and the manual skill of the operator in removing the excess powder.
Finally, the Research and Industrial Corporation of Russia, "TENAKON," describes a device entitled "A Tool for Solid Body Open Porosity Measurement." TENAKON describes a method by which a sample is placed on a vertically moveable piston positioned within a cylinder. A free flowing powder of some sort is then dumped on top of the sample. The cylinder is capped with a cover containing an electrical interrupter switch. The piston moves up and presumably compresses the free-flowing powder until the piston drive is interrupted. No attempt is made to distribute the material around the sample. The volume confined within the cylinder defines the volume of the sample once the free-flowing powder volume is subtracted. It is understood that such a device may be accurate for flat-bottomed samples, but not particularly accurate for crushed or irregular objects.
What is needed, therefore, is a method and apparatus for the measurement of envelope density that provides reliable, reproducible results. These results should be superior to those found with the use of other fluids or known dry medium methods and should not require tedious sample manipulation. The method and apparatus must be easy to use, employ non-hazardous materials, and be non-destructive to the object being tested.